ICP4 is both an activator and repressor of transcription depending on how it interacts with a viral promoter and the cellular transcription machinery. It is absolutely required for viral growth largely because it functions to activate the transcription of most viral genes to levels required for the ultimate production of progeny virions. The requirement for the repressor function of ICP4 is less well defined. While ICP4- repressible genes have been identified, the contribution of this function to the viral life cycle is not well understood. There are regions amino terminal to the DNA binding domain of ICP4 that are involved in both activation and repression of transcription and are conserved among the ICP4 analogs of 1-herpesviruses. In the context of the intact ICP4 molecule, these regions contribute little to viral growth or gene expression in tissue culture or in non-neuronal cells in vivo. However in the absence of the carboxyl-terminal activation region, mutants in the amino terminus are defective in activation and/or repression. This suggests that multiple regions of the ICP4 molecule contribute to its functions by specifying activities that may be redundant in some cell types and not others. In addition conserved regions that are not important for growth in one cell type may specify an activity that is uniquely required in another cell type. Studies from our lab suggest that there are domains of ICP4 that are required in neuronal cells and not other cell types. Studies from other labs suggest that the productive viral gene expression in neurons differs from that in non-neuronal cells. This is a very understudied aspect of HSV biology that may be crucial for our understanding of how HSV enters and reactivates from latency. The goals of this exploratory project are to identify conserved domains of ICP4 that are dispensable in cell culture, but are required for aspects of the virus life cycle in neurons or in vivo, and to begin to elucidate the molecular basis for the requirements in terms of virus gene expression and known ICP4 activities. These studies may also shed light on the basis for the different patterns of gene expression observed in neurons.